Asymmetric Ionospheric Fluctuations Over the Circum‐Pacific Regions Following the January 2022 Tonga Volcanic Eruption.

The Hunga Tonga‐Hunga Ha'apai volcanic eruption on 15 January 2022 had a significant impact on the ionosphere‐thermosphere system, resulting in large‐scale ionospheric irregularities with longitudinal and latitudinal asymmetries. Multiple instruments recorded these irregularities, indicating the propagation of a westward wave at an average velocity of 354 ± 8 m/s, which led to plasma irregularities of 0.2 TECu/min. Conversely, an eastward‐propagating wave was detected on the Pacific's east coast, traveling at a speed of 348 ± 6 m/s, with a corresponding decrease in plasma fluctuations to 0.1 TECu/min. In Asia, noticeable plasma irregularities appeared within a few hours after the eruption, and the maximum speed exceeded 1,100 m/s, which cannot be explained by the acoustic wave model. There was also a significant latitudinal asymmetry of ionospheric disturbances in the Asian‐Oceania sector, with the plasma density around Oceania depleted by 2–3 orders of magnitude within the altitudes of ∼150–575 km, while the ion density over Asia was enhanced by 1–2 orders of magnitude, and was uplifted ∼50 km. The plasma temperature was proportional to ion density, indicating the ion temperature reduced ∼500 K and increased 100–200 K around Oceania and Asia, respectively. The equatorial electric field, vertical E × B drifts and thermospheric O/N2 density ratio also fluctuated significantly following the eruption, indicating the redistribution of charged particles due to the magnetic field mapping effect, which was the main contributor to the asymmetries observed. Plain Language Summary: The Tonga eruption sent huge amounts of atmospheric waves around the world and a plume of ash into the atmosphere. As the largest atmospheric explosion in recorded history, it attracted scientists' attention. We focus on the ionospheric response to the eruption over the Circle‐Pacific regions using multiple space and ground based techniques. The results indicate the plasma irregularities triggered by the eruption were asymmetric in longitudinal and latitudinal directions. Propagation velocity and magnitude of plasma irregularities on the west coast of the Pacific were higher than those on the east coast. Notably, in Asia, plasma irregularities were observed much earlier than the arrival of volcanic‐induced atmospheric waves, with a maximum speed of over 1,100 m/s. The eruption also triggered total electron content depletion, with a reduction of 2–3 orders of magnitude around Oceania. At the same time, there was a significant increase in plasma density in the Asian sector, with an increment of 1–2 orders of magnitude. The fluctuations in ion temperature, equatorial electric field, vertical E × B drifts, and thermospheric O/N2 density ratio were also observed following the eruption, and the redistribution of charged particles associated with the magnetic field mapping effect was identified as the primary driver of these asymmetries. Key Points: Propagation velocity and magnitude of plasma irregularities on the west coast of the Pacific were higher than those on the east coastVolcanic eruption resulted in a depletion of plasma density and temperature around Oceania, while enhancing them in the Asian sectorRedistribution of charged particles caused by the mapping effect has been identified as the primary driver of ionospheric asymmetries [ABSTRACT FROM AUTHOR]